In general, HID lamps are operated using a low-frequency signal, preferably using an approximately sinusoidal signal at a frequency of preferably between 100 Hz and 500 Hz. For starting purposes, high-voltage pulses are superimposed on the low-frequency signal.
Certain HID lamp types are started using a radiofrequency high-voltage signal and then operated using a low-frequency signal which is preferably approximately rectangular and is between 50 and 500 Hz. One problem is the fact that the lamp may have a rectifying effect after starting, this being observed particularly frequently in the case of lamps having a short distance between the electrodes and/or thin electrodes. This rectification effect has proved to be very damaging for the lamp life. A further complication is the fact that the rectification effect disappears only slowly, or even not at all, during low-frequency operation.
U.S. Pat. No. 6,518,712, in which a generic driver circuit and a generic method are disclosed, cf. in particular FIGS. 4 and 8, therefore describes radiofrequency operation for a short period of time even after starting. This so-called RF delay is set permanently, which leads to two undesirable consequences: if the RF delay is set to be too short, the rectification effect continues to occur. If the RF delay is set to be too long, the lamp has already built up pressure which leads to acoustic resonances having the associated, known disadvantages.